Counter-flow expanding channels for enhanced two-phase heat removal

ABSTRACT

A structure for cooling an integrated circuit. The structure may include; an interposer cold plate having at least two expanding channels, each expanding channel having a flow direction from a channel inlet to a channel outlet, the flow direction having different directions for at least two of the at least two expanding channels, the channel inlet having an inlet width and the channel outlet having an outlet width, wherein the inlet width is less than the outlet width.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with United States Government support underDARPA Agreement No. FA8650-14-C-7466. THE GOVERNMENT HAS CERTAIN RIGHTSIN THIS INVENTION.

BACKGROUND

The present invention generally relates to cooling chip packages, andmore particularly to cooling chip stacks by employing an onboard coolingstructure.

In general, it is important to cool semiconductor chips, such asprocessor chips, to maintain reliable operation, reduce leakagecurrents, and prevent thermal damage to electronic components. It ismore problematic and difficult to implement effective mechanisms forcooling 3D chip stacks as compared to singular chips, and the ability toefficiently cool a chip stack can limit the height and total power of achip stack. Common cooling techniques for chip stacks include the use ofhigh-performance water cooling systems on a backside of the chip stack,but this technique may not be adequate for a stack structure with manychips or a chip stack having a high-power chip on the bottom of thestack. While a water-cooled thermal interposer can be used at the bottomof the chip stack, this structure is difficult to integrate and mayrequire isolation of thru silicon vias (TSVs) from the liquid coolantthat is used. If a dielectric fluid is used as the coolant, isolation ofthe TSVs is not required. However, with single phase cooling, theperformance of dielectric fluids is inferior to water.

Other cooling techniques include two-phase cooling in which a liquidcoolant having a relatively low boiling point is used (e.g., liquidwhich evaporates at an operating temperature of the chips being used).With two-phase cooling in closed channels, the heated liquid evaporatesto create an annular flow whereby a thin liquid film (evaporation layer)is present on the surfaces being cooled, and heated evaporated coolantflows through confined channels to outlet ports. As such, compared topure liquid (e.g., single phase) cooling techniques, two-phase coolingcan provide greater cooling ability using a much lower volume of coolantfluid, lower coolant mass flow rates and lower operating pressure drop.Advantages of two-phase cooling include the ability to select theboiling temperature of the coolant or use an expansion valve forrefrigeration.

Another cooling technique includes expanding radial channels to mitigatethe acceleration in the stream-wise direction of the fluid cavity.Accordingly, the pressure drop and the dry-out risk can be reduced inthe heat transfer cavity. However, a central hole is required in a chipstack to feed the refrigerant into the radial expanding channels.

SUMMARY

According to one embodiment of the present invention, a structure isprovided. The structure may include: an interposer cold plate having atleast two expanding channels, each expanding channel having a flowdirection from a channel inlet to a channel outlet, the flow directionhaving different directions for at least two of the at least twoexpanding channels, the channel inlet having an inlet width and thechannel outlet having an outlet width, wherein the inlet width is lessthan the outlet width.

According to another embodiment of the present invention, a structure isprovided. The structure may include: a single heat transfer structurethermally coupled to the chip, wherein the single heat transferstructure having two or more expanding coolant channels, the expandingcoolant channels having a flow direction from a channel inlet to achannel outlet, wherein the flow direction of one of the at least twoexpanding coolant channels is in a different direction than another oneof the at least two expanding coolant channels, and wherein theexpanding channels have an inlet width less than an outlet width; and amanifold providing an inlet pathway to the channel inlet of the at leasttwo expanding coolant channels and an outlet pathway from the channeloutlet of the at least two expanding coolant channels.

According to another embodiment of the present invention, a method isprovided. The method may include: bonding the electronic device to aninterposer cold plate having at least two expanding channels, whereinthe at least two expanding channels each include an inlet width lessthan an outlet width; generating a first coolant flow direction within afirst expanding channel of one of the at least two expanding channels;and generating a second coolant flow direction within a second expandingchannel of another one of the at least two expanding channels, whereinthe first coolant flow direction and the second coolant flow directioninclude different flow directions.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description, given by way of example and notintended to limit the invention solely thereto, will best be appreciatedin conjunction with the accompanying drawings, in which:

FIG. 1 is a cross section view of a cooling structure, according to anexemplary embodiment;

FIG. 2 is a cross sectional top view of an interposer cold plate withexpanding channels, according to an exemplary embodiment;

FIG. 3 is a cross sectional top view of the interposer cold plate withexpanding channels and illustrates temperature zones in the expandingchannels, according to an exemplary embodiment;

FIG. 4 is a cross sectional top view of the interposer cold plate withexpanding channels and illustrates an average path of travel of coolantthrough the expanding channels, according to an exemplary embodiment;

FIG. 5 is a cross sectional top view of the interposer cold plate withexpanding channels and illustrates an alternative design for theexpanding channels, according to an exemplary embodiment;

FIG. 6 is a cross sectional top view of the interposer cold plate withexpanding channels and illustrates an alternative design for theexpanding channels, according to an exemplary embodiment;

FIG. 7 is a cross sectional top view of the interposer cold plate withexpanding channels and illustrates an alternative design for theexpanding channels, according to an exemplary embodiment;

FIG. 8 is a cross section view of a chip stack with multiple interposercold plates, according to an exemplary embodiment; and

FIG. 9 is a block diagram and graph illustrating a cooling loop,according to an exemplary embodiment.

The drawings are not necessarily to scale. The drawings are merelyschematic representations, not intended to portray specific parametersof the invention. The drawings are intended to depict only typicalembodiments of the invention. In the drawings, like numbering representslike elements.

DETAILED DESCRIPTION

Detailed embodiments of the claimed structures and methods are disclosedherein; however, it can be understood that the disclosed embodiments aremerely illustrative of the claimed structures and methods that may beembodied in various forms. This invention may, however, be embodied inmany different forms and should not be construed as limited to theexemplary embodiments set forth herein. Rather, these exemplaryembodiments are provided so that this disclosure will be thorough andcomplete and will fully convey the scope of this invention to thoseskilled in the art. In the description, details of well-known featuresand techniques may be omitted to avoid unnecessarily obscuring thepresented embodiments.

References in the specification to “one embodiment”, “an embodiment”,“an example embodiment”, etc., indicate that the embodiment describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

For purposes of the description hereinafter, the terms “upper”, “lower”,“right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, andderivatives thereof shall relate to the disclosed structures andmethods, as oriented in the drawing figures. The terms “overlying”,“atop”, “on top”, “positioned on” or “positioned atop” mean that a firstelement, such as a first structure, is present on a second element, suchas a second structure, wherein intervening elements, such as aninterface structure may be present between the first element and thesecond element. The term “direct contact” means that a first element,such as a first structure, and a second element, such as a secondstructure, are connected without any intermediary conducting, insulatingor semiconductor layers at the interface of the two elements.

In the interest of not obscuring the presentation of embodiments of thepresent invention, in the following detailed description, someprocessing steps or operations that are known in the art may have beencombined together for presentation and for illustration purposes and insome instances may have not been described in detail. In otherinstances, some processing steps or operations that are known in the artmay not be described at all. It should be understood that the followingdescription is rather focused on the distinctive features or elements ofvarious embodiments of the present invention.

The present invention generally relates to cooling chip packages, andmore particularly to cooling chip stacks by employing a two-phasecooling in a counter-flow arranged structure having expanding channels.Ideally, it may be desirable to fabricate an interposer cold plate and abackside cold plate that allows the heat dissipation from two sides ofthe chip stack, and with the advantages of expanding channels, withoutthe need for a hole going through the chip stack (as used in, forexample, radial expanding channels).

One way to fabricate the two-phase cooling structure having expandingchannels is to split a fluid inlet loop and a fluid outlet loop eachinto two or more loops, resulting in a counter-flow heat exchange. Oneembodiment by which to form the two-phase counter-flow cooling structureis described in detail below referring to the accompanying drawingsFIGS. 1-7.

With reference to FIG. 1, a demonstrative illustration of a structure100 having a counter-flow cooling structure with expanding channels isprovided, according to an embodiment. More specifically, an inlet loopsupplies a fluid into two or more sides of an interposer cold plate 112and a backside cold plate 114. The interposer cold plate 112 may be usedalone or in combination with the backside cold plate 114 depending oncooling requirements. FIG. 1 is a cross sectional view of structure 100.

The structure 100 may include internal components, such as a chips stack106, the interposer cold plate 112 and the backside cold plate 114. Thechip stack may be cooled on one side by the interposer cold plate 112and an opposite side by the backside cold plate 114. A manifold 104 maycover a top and side portion of the internal components, and themanifold 104 and the internal components may be bonded to a laminate 102on a bottom portion.

The chip stack 106 may include one or more semiconductor chipsvertically stacked. In an embodiment, the chip stack 106 includes threechips joined by minibumps 118. The minibumps 118 may allow forstructural support as well as electrical connection between chips withinthe chip stack 106. Additionally, the minibumps 118 may provide a meansof bonding the chip stack 106 to the interposer cold plate 112.

The interposer cold plate 112 may be any material known in the art, suchas, for example, silicon and may be formed by bonding two siliconhalves. The interposer cold plate 112 has two or more expanding fluidchannels 113 (may also be referred to as fluid cavities). Each expandingfluid channel 113 may have a channel inlet and a channel outlet ondifferent sides of the interposer cold plate 112. The two or moreexpanding fluid channels 113 may be oriented in a way to allow forcounter-flow (i.e., a flow direction from channel inlet to channeloutlet is oriented in a different direction than the flow direction ofan adjacent expanding fluid channel 113).

In the illustrated embodiment, the backside cold plate 114 is used, suchthat the back side cold plate 114 and the interposer cold plate 112 coolthe opposite side of the chip stack 106. The backside cold plate 114 maybe any backside cold plate used in the art, or may be similar to theinterposer cold plate 112 (i.e., having two or more expanding channels115).

In the embodiment having a backside cold plate 114 and the interposercold plate 112, a two-sided cooling system can be provided to the chipsstack 106. The two-sided cold plate concept can nearly double the heatremoval from the chip stack 106. For example, the backside cold plate114 may be a metal or silicon cold plate that is attached to the chipstack 106 using a thermal interface material (TIM) 107 to allow for heattransfer from the chip stack 106. The backside cold plate 114 may alsobe used to preheat liquid intended to enter further cooling elements,such as the interposer cold plate 112. The expanding fluid channels 113,115 may be defined by channel walls 122 (illustrated in FIG. 2).

The chip stack 106 may be thermally connected to TSVs 108, where theTSVs 108 pass through the interposer cold plate 112. In addition to thethermal properties of the TSVs 108, the TSVs 108 may allow for bothstructural support and electrical connection to the chips stack 106. TheTSVs 108 may electrically connect the chip stack 106 to the laminate 102through solder balls 101 (e.g. micro BGA or C4). The solder balls 108may also provide a means of bonding the interposer cold plate 112 to thelaminate 102. The laminate 102 can be, for example, an organic build-upsubstrate or a ceramic single or multi-chip module.

A coolant (a liquid) may be supplied to each of the channel inlets ofexpanding fluid channels 113. A nozzle (not illustrated) may be used toprovide flow restriction of the coolant to the expanding fluid channels113. The coolant may pass between the TSVs 108 and provide the two-phasecooling to cool the chip stack 106. The coolant may exit the expandingfluid channels 113 as a liquid/vapor combination at the channel outlets.The coolant (in liquid form) is represented by solid black arrows and asoutlined arrows after passing through the expanding fluid channels 113.The coolant may be supplied to the channel inlet through the manifold104 starting at an inlet port and may exit the manifold 104 at an outletport.

With reference to FIG. 2, a demonstrative illustration of the interposercold plate 112 and the expanding fluid channels 113 is provided,according to an embodiment. FIG. 2 is a top view of the interposer coldplate 112 taken along cross sectional line A-A of FIG. 1. In theexemplary embodiment, six expanding fluid channels 113 are illustratedin a staggered arrangement, but any number of expanding fluid channels113 in a variety of arrangements may be used. It should be noted, theTSVs 108 are only illustrated in one of the expanding fluid channels 113for illustrative purposes and may be present in more than one, or all,expanding fluid channels 113.

A coolant flows into the expanding fluid channels 113 from the channelinlets, passing through the expanding fluid channels 113, and exiting atthe channel outlets. The expanding fluid channels 113 having dimensionsdefined by the location of the channel inlets/outlets and the channelwalls 122. The coolant may enter an opening into the expanding fluidchannels 113 having an inlet width (iw) and may exit the expanding fluidchannels 113 at an opening having an outlet width (ow). To achieve theexpanding nature of the expanding fluid channels 113, the inlet width(iw) may be less than the outlet width (ow).

Having expanding channels can result in a reduction of vapor-liquidacceleration, compared to straight channels. This reduction invapor-liquid acceleration, in turn, reduces pressure drops and improvesthe critical heat flux. A benefit of expanding channels is that the flowspeed (and pressure drop) is reduced as the channel cross-sectionincreases, thus maintaining an annular regime with a relatively thickevaporating film along surfaces of flow cavities. Another benefit mayinclude the elimination of an inlet hole needing to be fabricated in aradial design. A further benefit is a reduction in the flowinstabilities observed in non-expanding channels, such as hot spotformation.

In other embodiments of the invention, the expanding channels 113 may bepatterned or textured to reduce surface superheat. Moreover, theexpanding channels 113 may be varied in size to correspond withdifferent power densities by chip layer or location on a given chip(FIG. 5). In another embodiment, the expanding channels 113 can vary inconfiguration relative to one another to specifically direct liquidcoolant/refrigerant to hot spot locations (FIG. 6) and may be suppliedfrom more than two sides of the interposer cold plate 112 (FIG. 7). Aflow channel network can be designed to focus multiple channels to oneor more hot spot locations.

With reference to FIG. 3, a demonstrative illustration of the expandingfluid channels 113 in the interposer cold plate 112 is provided,according to an embodiment. More specifically, the heat distribution isillustrated as the coolant passes through the expanding fluid channels113.

The heat distribution of the two-phase liquid coolant is shown as shadesof gray, where the light gray represents a lower temperature (e.g., aliquid) and the darker gray represents a higher temperature (e.g., aliquid/vapor combination). As illustrated, cooling effects aredistributed more evenly, from inlet to outlet, as compared to knowndesigns. As the liquid coolant flows through the expanding fluidchannels 113, the liquid evaporates resulting in a variety of flowpatterns, for example, annular, turbulent, or any other flow pattern.

With reference to FIG. 4, a demonstrative illustration of the expandingfluid channels 113 in the interposer cold plate 112 is provided,according to an embodiment. More specifically, the figure illustratesthe flow path of the coolant as it passes through the expanding fluidchannels 113. The dotted lines represent a few flow paths depending onthe shape of the expanding fluid channels 113. The dashed linerepresents an average flow path through each expanding fluid channel113. Some of the average flow path lines may be parallel to one another,while other average flow path lines may not be parallel.

With reference to FIG. 5, a demonstrative illustration of the expandingfluid channels 113 in the interposer cold plate 112 is provided,according to an embodiment. More specifically, an alternative embodimentof the expanding fluid channels 113 is illustrated. As stated above, theexpanding fluid channels 113 can be designed with varied inlet andoutlet sizes to account for hotspot areas. As the ratio of inlet width(iw) to outlet width (ow) gets closer to 1:1, the vapor-liquidacceleration may increase while providing a lower temperature toportions of the chip stacks that are closer to the outlet. In turn, whenthe ratio of inlet width (iw) to outlet width (ow) gets farther from1:1, the vapor-liquid acceleration may decrease, but the liquid coolantassociated with the given inlet will have to cool a larger area. Theamount of coolant provided to each zone relative to the other zones canalso be modulated by selecting appropriate nozzle dimensions.

With reference to FIG. 6, a demonstrative illustration of the expandingfluid channels 113 in the interposer cold plate 112 is provided,according to an embodiment. More specifically, an alternative embodimentof the expanding fluid channels 113 having curved channel walls 122 maybe used. As chip layouts become more complex, the expanding fluidchannels 113 can be designed to provide more efficient pathways betweenthe inlet and outlet openings given an understanding of the expectedpower distribution/map.

With reference to FIG. 7, a demonstrative illustration of the expandingfluid channels 113 in the interposer cold plate 112 is provided,according to an embodiment. More specifically, an alternative embodimentof the expanding fluid channels 113 having inlet and outlet pathways onmore than two sides of the interposer cold plate 112 is possible. Aschip layouts become more complex, the expanding fluid channels 113 canbe designed to provide more efficient pathways between the inlet andoutlet openings.

With reference to FIG. 8, a demonstrative illustration of an alternativestructure 200 is provided, according to an embodiment. Morespecifically, the structure 200 is an alternative layout having multiplechips or chip stacks 206 between more than two interposer cold plates212. It should be noted, the interposer cold plates 212 may be similarto the interposer cold plate 112 described in reference to FIG. 1.

With reference to FIG. 9, a demonstrative illustration of a pumpedtwo-phase loop 300 is provided, according to an embodiment. Morespecifically, if a pump 302 is used, the coolant would generally need tobe entirely liquid to limit cavitation and avoid vapor at channel inletsand orifices. It should be noted, a pressure and temperature chart hasbeen provided below a block diagram to illustrate the temperature andpressure changes as the coolant flows through the pumped two-phase loop300.

The pumped two-phase loop 300 can include the coolant flowing throughthe pump 302, to a pre-heater 304, module 306, and a condenser 308. Thepre-heater 304 may help with heat transfer efficiency when the coolantpasses through the module 306. The pre-heater 304 may be the backsidecold plate 114. The module 306 may be a structure similar to structure100 (described in reference to FIG. 1). The condenser 308 may condensethe coolant from vapor to liquid, where the liquid coolant is thenresupplied to the pump 302.

The slight sub-cooling needed before the coolant passes through the pump302 is represented by dT_(sub1). Pressure lifts by the pump 302increases the sub-cooling to dT_(sub2). If the module 306 includes chipcooling, two-phase heat transfer may be required across the entire chip.Hence, coolant pre-heating may be required to reduce sub-cooling todT_(sub3). Flashing may occur after the pressure drop across anintegrated nozzle.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the invention.The terminology used herein was chosen to best explain the principles ofthe embodiment, the practical application or technical improvement overtechnologies found in the marketplace, or to enable others of ordinaryskill in the art to understand the embodiments disclosed herein.

What is claimed is:
 1. A structure for cooling an integrated circuitcomprising: an interposer cold plate having at least two expandingchannels, each expanding channel having a flow direction from a channelinlet to a channel outlet, the flow direction having differentdirections for at least two of the at least two expanding channels, thechannel inlet having an inlet width and the channel outlet having anoutlet width, wherein the inlet width is less than the outlet width. 2.The structure of claim 1, wherein the at least two expanding channelsare arranged in an alternating arrangement, wherein directly adjacentexpanding channels have opposing flow directions.
 3. The structure ofclaim 1, wherein the interposer cold plate has a channel inlet on fourdifferent sides of the interposer cold plate.
 4. The structure of claim1, wherein the at least two expanding channels are in a counter-flowarrangement, wherein the channel inlet is directly next to the channeloutlet of a directly adjacent expanding channel.
 5. The structure ofclaim 1, wherein an average flow direction of adjacent expandingchannels are not parallel.
 6. The structure of claim 1, wherein theintegrated circuit is electrically connected to a substrate.
 7. Thestructure of claim 1, further comprising a backside cold plate, whereina preheated flow direction is from the backside cold plate to thechannel inlet.
 8. The structure of claim 1, wherein at least two or moreinterposer cold plates are separated by at least one electronic device.9. A structure for cooling a chip comprising: a single heat transferstructure thermally coupled to the chip, wherein the single heattransfer structure having two or more expanding coolant channels, theexpanding coolant channels having a flow direction from a channel inletto a channel outlet, wherein the flow direction of one of the at leasttwo expanding coolant channels is in a different direction than anotherone of the at least two expanding coolant channels, and wherein theexpanding channels have an inlet width less than an outlet width; and amanifold providing an inlet pathway to the channel inlet of the at leasttwo expanding coolant channels and an outlet pathway from the channeloutlet of the at least two expanding coolant channels.
 10. The structureof claim 9, wherein the at least two expanding coolant channels arearranged in an alternating arrangement, wherein directly adjacentexpanding coolant channels have opposing flow directions.
 11. Thestructure of claim 9, wherein the single heat transfer structure has afluid inlet on four different sides.
 12. The structure of claim 9,wherein the at least two expanding coolant channels are in acounter-flow arrangement and the flow direction of adjacent expandingcoolant channels are in opposing directions.
 13. The structure of claim9, wherein the flow direction of each expanding coolant channel has anaverage flow direction, and the average flow direction of all adjacentexpanding coolant channels are not parallel.
 14. The structure of claim9, wherein the chip is electrically connected to a substrate through athrough silicon via (TSV), and wherein the TSV provides structuralsupport for the chip.
 15. The structure of claim 9, further comprising abackside cold plate, wherein a preheated flow direction is from thebackside cold plate to the channel inlet.
 16. The structure of claim 9,wherein at least two or more of the single heat transfer structures areseparated by at least one chip.
 17. A method of cooling an electronicdevice comprising: bonding the electronic device to an interposer coldplate having at least two expanding channels, wherein the at least twoexpanding channels each include an inlet width less than an outletwidth; generating a first coolant flow direction within a firstexpanding channel of one of the at least two expanding channels; andgenerating a second coolant flow direction within a second expandingchannel of another one of the at least two expanding channels, whereinthe first coolant flow direction and the second coolant flow directioninclude different flow directions.
 18. The method of claim 17, whereinthe electronic device is electrically connected to a substrate with athrough silicon via (TSV).
 19. The method of claim 18, wherein the TSVprovides structural support for the electronic device.
 20. The method ofclaim 17, further comprising a backside cold plate, wherein a preheatedflow direction is from the backside cold plate to the channel inlet.